Hydraulic stimulation (known as fracture or frac) is used in shale gas wells that use slick water frac to stimulate, or fracture the shale. Slick frac water (also referred to herein as “frac water”) includes either fresh water, or a blend of fresh and treated ‘flow back’ water. Treated flow back water is water that had been previously pumped into the shale formation, and was returned to the surface during the testing and production periods. The flow back water generally contains dissolved and suspended solids, typically having high concentrations of dissolved salts and other minerals in the form of suspended fines. Flow back water is stored in large pressure vessels, such as tanks, prior to reuse.
Any sand, salt, or other solid concentrations in the flow back water can cause problems with water reuse during well testing, as well as additional wear and tear on pumping equipment. Therefore it is best if the solids in the flow back water are separated before leaving the tank.
During the process of “flowing” the well, after pumping the sand and water into the well, the frac equipment is removed and the well is ‘flow tested’, meaning the well head is opened up and the gas and water returned to the surface under pressure. Measurements are then taken, and the quality of the well can be extrapolated during this flow period. After the testing is completed, the well is tied in and production begins. The gas and water returned from the well is directed to a tank wherein the primary separation of water, sand and gas occurs. The common tanks used to store the flow back water are large upright cylindrical tanks, typically having a 400 bbl (60 m3) capacity and a door allowing workers to access the interior of the tank
The liquid in the tank is a mixture of water, salts dissolved in water, and fines suspended in the same. The size of fines varies from sand grain size (from sand used as propant during the frac phase) to finer solids from the reservoir, and could range from less than 1 micrometer to greater than 300 micrometers. Depending on the flow rate from the well, size of vessel, water to gas ratio, viscosity etc., there are various retention times of water in the tank. The retention time in the tank, in combination with other physical properties of the flowback mixture such as temperature, viscosity, turbulence etc., controls the degree of separation of the variously sized solids.
The flow back water is typically directed to a pressure test vessel first. The primary function of the pressure vessel tank is to separate the gas from the water and sand grain size fines, or solids, and redirect the gas to the well flare. The sand grain size solids are periodically cleaned from the pressure vessel by stopping the flow of water from the well, and using a vacuum truck, or in some cases, by opening the door of the pressure vessel and shoveling the contents outside, or into the suction of a vacuum truck suction hose. The water and finer solids mixture is directed to other collecting tanks, such as standard oilfield 400 bbl sloped bottom tanks, large reservoirs in the form of C-ring from Westeel, settling tanks, or any other form of collecting and storing flowback water. Alternatively, the water may be pumped directly through a pipeline to some other system for reconditioning and subsequent reuse or disposal.
Since there are still some solids in the water after it leaves the pressure vessel and the subsequent tanks are significantly larger than the pressure vessel (and hence have a higher retention time) some of the solids deposit in the bottom of these settling tanks. These solids deposits build up inside the settling tank and removal of them is typically done by connecting a vacuum truck to one of the ports on the tank and sucking the solids out, or alternatively by using a door and cleaning out the solids with the use of a worker and shovel. Typically the tanks are filled in the front and discharged in the back, and in other cases they are cascaded from one tank to the next and so on. This causes the solids that are dropped in to the bottom of the tank to get disturbed frequently by the water coming from the inlet of the tank, and the solids then get carried to the discharge of the tank and subsequently into the next tank, pump or the flow line.